1. Field of the Invention
This invention relates to a toner for an image-forming apparatus usable in copying machines, printers, plotters, facsimile machines and so forth.
2. Description of the Related Art
As one of conventional image-forming apparatus, for example, U.S. Pat. No. 3,689,935 discloses an apparatus in which, using an electrode member with a plurality of openings (hereinafter "apertures") formed therein, the pass of toner particles through the apertures is controlled in accordance with image data so that an image is formed on a recording medium by the use of the toner particles having passed through them.
Stated specifically, this image-forming apparatus comprises:
an aperture electrode member constituted of a thin flat sheet formed of an insulating material, a reference electrode continuously formed on one side of the flat sheet, a plurality of control electrodes insulated from one another, formed on the other side of the flat sheet, and a plurality of apertures which are so formed as to run through the flat sheet, the reference electrode and the control electrode for each control electrode and are arranged at least in a row; PA1 a means for selectively providing between the reference electrode and the control electrode a potential difference in accordance with image data; PA1 a means for feeding electrostatically charged particles toward the apertures in such a manner that the pass of the particles through the apertures is controlled according to the potential difference; and PA1 a means for positionally adjusting (or registering) a recording medium in the flow path of the toner particles so that the recording medium and the aperture electrode member can move in a relative fashion.
U.S. Pat. No. 4,743,926, U.S. Pat. No. 4,755,837, U.S. Pat. No. 4,780,733 and U.S. Pat. No. 4,814,796 also disclose an image-forming apparatus in which such an aperture electrode member is so provided that its control electrodes face a recording medium side and its reference electrode faces a toner feeding side.
In contrast, U.S. Pat. No. 4,912,489 disclose an image-forming apparatus in which such an aperture electrode member is so provided that its reference electrode faces a recording medium side and its control electrodes face a toner feeding side so that the voltage applied to the control electrodes can be made lower by about 1/4 than the image-forming apparatus disclosed in the above U.S. Patents.
Here, the time at which the toner is not allowed to pass through the apertures so that the toner particles are not made to adhere onto the recording medium, i.e., the point of time at which white background areas of an image are formed thereon is hereinafter called "off-time", and the reverse instance, i.e., the point of time at which the toner is allowed to pass through the apertures so that dots are formed by the toner on the recording medium is hereinafter called "on-time".
The voltage applied to the control electrodes is also hereinafter called "control voltage". Thus, "control voltage at the on-time" is meant to be a voltage applied to the control electrodes in order to form the toner dots on the recording medium by allowing the toner particles to adhere onto the recording medium. In reverse, "control voltage at the off-time" is meant to be a voltage applied to the control electrodes in order to form a white background of an image on the recording medium by allowing the toner not to pass through the apertures. The difference in voltage between the control voltage at the on-time and the control voltage at the off-time is called "drive voltage". Also, an aggregate of the toner dots disposed at any desired position on the recording medium by using the image-forming apparatus constituted as described above is called "toner image". Thus, the toner image is meant to be an image formed by selectively arranging the toner dots on the white background that is a ground of the recording medium. Arrangement of dot areas and white background areas is regarded as the toner image.
In the conventional image-forming apparatus as described above, however, the control electrodes are driven by circuit devices such as ICs and hence the drive voltage may preferably be set as low as possible. In order to use practical and inexpensive ICs, the control voltage must be set within 50 V, whereas the relationship between control voltage and image density (hereinafter "control voltage characteristics") stands as shown by a broken-line curve in FIG. 5A. Accordingly, whatever control voltages are set at the on-time and at the off-time within the range where the control voltage of 50 V is maintained, it has been substantially impossible to attain image-forming conditions under which a sufficient density of 1.5 or above as a value of reflection density at the on-time and a good white background free of fog, having a reflection density of 0.07 or below, at the off-time can be achieved at the same time.
The control voltage characteristics shown in FIGS. 5A and 5B are as obtained in an instance where a negatively chargeable toner is used as the toner.
Stated specifically, in the control voltage characteristics shown in FIG. 5A, the slope on the high-voltage side is steep and on the other hand the slope on the low-voltage side is gentle. Hence, a voltage of +65 V or above is necessary in order to attain the density required at the on-time and, taking account of safety factors, a control voltage of about +70 V must be actually applied. Assuming this voltage as a standard and in an instance where the control voltage is made lower than this voltage, the off-state is not attained even if the control voltage is made lower by 50 V than that at the on-time so as to be +20 V, because the slope of the control voltage characteristics on the low-voltage side is gentle. In order to surely provide the off-state, the control voltage must be made lower than -20 V, so that the drive voltage reaches 90 V.
Incidentally, ideal control voltage characteristics are as shown by a solid line in FIG. 5B, where the density can be binarily controlled at a certain voltage threshold value. Under such conditions, the drive voltage can be made very low. It, however, actually stands as shown by the broken line in FIG. 5A. The greatest reason therefor is that the toner fed to a toner flow control means varies in charge quantity.
More specifically, the electrostatic force acting on the toner differs as a matter of course when the charge quantity differs. Hence, when the control voltage characteristics are set as shown in FIG. 5B in respect of a toner having a charge quantity in a certain narrow range, the voltage threshold value differs as a matter of course in respect of a toner having a charge quantity different from that, so that a value shifted in parallel in the lateral direction in FIG. 5B is obtained. Thus, a toner having a charge quantity distribution has such control voltage characteristics that the curve of FIG. 5B concerning the different charge quantity is overlapped with the charge quantity distribution, and consequently, has the characteristics as shown by the broken line in FIG. 5A.
The charge quantity distribution of such a toner is partly ascribable to the distribution of particle size, but the greatest reason therefor is that the toner is not uniformly saturation-charged. More specifically, the reason is that the toner has a broad distribution in type including a toner well saturation-charged and even a toner almost not charged. Hence, it is most preferable for the toner to be uniformly saturation-charged. For this end, the toner may be triboelectrically charged for a longer time so that the toner can have more opportunities for triboelectric charging.
However, in the conventional image-forming apparatus, the charging of toner by a toner feeding means is commonly carried out by bringing the toner into friction between a toner carrying member and a toner feeding means which are constituents of the toner feeding means, and between the toner carrying member and a toner layer control blade. Since, however, no satisfactory results have been obtained by only these means, the present applicant has proposed in Japanese Patent Application Laid-open No. 6-155798 an image-forming apparatus so constructed that, in addition to these means, the toner carrying member is pressed against the electrode member in its vicinity where the apertures are formed.
In this image-forming apparatus, the toner can be triboelectrically charged also between the apertures and the toner carrying member, and hence the charging of the toner reaches a saturation value. Moreover, because of slidable movement imparted to the toner at the time of this charging, the toner tumbles on the surface of the toner carrying member, so that the attractive force due to the electrostatic image force and so forth acting between the toner and the surface of the toner carrying member is weakened and the ejection of toner from the apertures at the on-time is promoted. Furthermore, the distance between the surface of the toner carrying member and the control electrodes of the electrode member, which provides the part where the toner is fed to the apertures, become close to the extent of the particle size of the toner, so that the effect of preventing the toner flow from passing through the apertures at the off-time can be improved to bring about the effect of making the control voltage lower.
However, this image-forming apparatus has had the following problems: The pressure acting between the electrode member and the toner carrying member brings about an increase in stress such as shear force applied to the toner, which may cause the toner to thermally melt-adhere to the electrode member with lapse of time of image formation on a few sheets to tens of sheets of A4-size paper, and the resultant molten deposit may obstruct the feeding of toner to apertures, resulting in a decrease in density of images formed, or in a worst case, making it impossible to form images.